This invention relates to methods of joining aluminum by brazing and, more particularly, to a method for brazing aluminum and its alloys without the use of a flux. It also relates to manufacture of large aluminum assemblies such as heat exchangers by fluxless brazing.
Most aluminum structures which are made up of parts joined by brazing are at present prepared by furnace brazing or dip brazing using a flux. Furnace brazing is generally used for relatively small articles, while dip brazing is used for relatively large objects.
For example, in manufacturing a large heat exchanger by dip brazing, the assembled structure is preheated to about 550.degree. C. in a hot air furnace, then immersed in the flux at about 590.degree. to 620.degree. C. The flux removes the oxidized film on the surface of the aluminum members and prevents reoxidation of the surface. At the same time, the assembly is rapidly heated to the brazing temperature by virtue of the large heat capacity of the flux and brazed into one unit. Large heat exchangers having dimensions up to about 1200 mm wide, 1200 mm high and 7000 mm long are manufactured by this method.
Since, however, the fluxes used for brazing consist principally of chlorides and fluorides, they are very corrosive toward aluminum. Hence, careful cleaning is required after brazing with accompanying expenditure of money and time. In view of this, brazing without using flux, or fluxless brazing, is beginning to receive attention.
Fluxless brazing is carried out either in vacuum or in an inert gas atmosphere. The structure to be brazed is heated in a furnace, by radiation in the case of heating in vacuum, by radiation and convection in the case of an inert gas atmosphere. The brazing cycle is generally completed within one hour when the object to be brazed is small, such as a condenser or an evaporator of an automobile air conditioner. When large heat exchangers as described above, or the like, are to be brazed, however, it may take as long as 24 hours or more to attain the brazing temperature of 590.degree. to 610.degree. C., because the large heat capacity of the flux is not available as in dip brazing.
In a brazing process which requires a long heating period, as in the case of manufacturing a large structure by fluxless brazing, the behavior of Si, Mg or similar elements in the brazing alloy must be considered.